arXiv subjects

Bogeun Gwak

Publications and source records attributed to Bogeun Gwak.

At least 37 records · Page 2Linked to original sources

arXiv2022

Quasinormal Modes in Near-Extremal Spinning C-Metric

We investigate the quasinormal modes of the spinning C-metric with a massless scalar field conformally coupled with gravity. Conformal transformation is employed to separate and subsequently solve the massless scalar field equations. As the outer and acceleration horizons approach each other, the potential term of the field equation is reduced to the Pöshl-Teller potential because the acceleration horizon is similar to the cosmological horizon of de Sitter spacetime. Finally, we obtain the analytical quasinormal frequency at which the decay rate is quantized.

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arXiv2022

Weak Cosmic Censorship Conjecture in Myers-Perry Black Hole with Separability

We investigate the weak cosmic censorship conjecture in Myers-Perry black holes with arbitrary rotations in general dimensions based on the scattering of a massless scalar field. From the fluxes of the scalar field flowing into the black hole, the changes in mass and angular momenta of the black hole are obtained. However, the extremal and near-extremal black holes with the aforementioned changes are still black holes in the final state. Hence, the conjecture is valid for our investigation. Furthermore, we analyze the changes in the black hole from a thermodynamic perspective to highlight that the laws of thermodynamics support the conjecture.

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arXiv2022

Violation of bound on chaos for charged probe in Kerr-Newman-AdS black hole

We investigate the conjectured bound on the Lyapunov exponent for a charged particle with angular motion in the Kerr-Newman-AdS black hole. The Lyapunov exponent is calculated based on the effective Lagrangian. We show that the negative cosmological constant reduces the chaotic behavior of the particle, namely, it decreases the Lyapunov exponent. Hence, the bound is more effective in the AdS spacetime than in the flat spacetime. Nevertheless, we find that the bound can be violated when the angular momenta of the black hole are turned on. Moreover, we show that in an extremal black hole, the bound is more easily violated compared to the nonextremal black hole.

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arXiv2022

Bound on the Lyapunov exponent in Kerr-Newman black holes via a charged particle

We investigate the conjecture on the upper bound of the Lyapunov exponent for the chaotic motion of a charged particle around a Kerr-Newman black hole. The Lyapunov exponent is closely associated with the maximum of the effective potential with respect to the particle. We show that when the angular momenta of the black hole and particle are considered, the Lyapunov exponent can exceed the conjectured upper bound. This is because the angular momenta change the effective potential and increase the magnitude of the chaotic behavior of the particle. Furthermore, the location of the maximum is also related to the value of the Lyapunov exponent and the extremal and non-extremal states of the black hole.

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arXiv2021

Radiation Spectra and Effective Temperatures in Higher-Dimensional Charged-de Sitter Black Holes

In this study, a scalar field propagating in a higher-dimensional Reissner-Nordström-de Sitter black hole is investigated. Scalar fields are assumed to have non-minimal coupling to the brane or bulk scalar curvature. Five different {definitions} of black hole temperatures are discussed: temperature based on surface gravity, Bousso-Hawking temperature, and three effective temperatures. The greybody factors of minimally and non-minimally coupled scalar fields on the brane and in the bulk are examined under the effect of particle and spacetime properties. The energy emission spectra of black holes are determined at various temperatures for both the brane and bulk channels. The energy emission rates at the Bousso-Hawking temperature are found to be dominant over those at other temperatures. The energy emission curves are suppressed by the presence of coupling parameters. Finally, the bulk-over-brane emission ratios are calculated. Notably, bulk dominance becomes possible for a certain definition of the temperatures and regime of the cosmological constant.

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arXiv2021

Weak Cosmic Censorship Conjecture in Kerr-Newman-(Anti-)de Sitter Black Hole with Charged Scalar Field

We investigate the weak cosmic censorship conjecture in extremal and near-extremal Kerr-Newman-(anti-)de Sitter black holes by the scattering of a massive scalar field with an electric charge. Under this scattering, the scalar field fluxes change the black hole state, as determined by the mass, angular momentum, and electric charge. The black hole may exceed its extremal condition because of these changes. However, we find that the black hole cannot be overcharged or overspun by the scattering. In particular, although the fluxes are closely associated with the asymptotic boundary conditions along the flat, anti-de Sitter, and de Sitter spacetimes, the weak cosmic censorship conjecture is valid for any scalar field boundary conditions. Moreover, the validity of the weak cosmic censorship conjecture is thermodynamically preferred for this scattering.

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arXiv2020

Thermodynamics of Warped Anti-de Sitter Black Holes under Scattering of Scalar Field

We investigate the thermodynamics and stability of the horizons in warped anti-de Sitter black holes of the new massive gravity under the scattering of a massive scalar field. Under scattering, conserved quantities can be transferred from the scalar field to the black hole, which change the state of the black hole. We determine that the changes in the black hole are well coincident with the laws of thermodynamics. In particular, the Hawking temperature of the black hole cannot be zero in the process as per the third law of thermodynamics. Furthermore, the black hole cannot be overspun beyond the extremal condition under the scattering of any mode of the scalar field.

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arXiv2020

Massive Scalar Perturbations on Myers-Perry-de Sitter Black Holes with a Single Rotation

This study investigates the stability of higher-dimensional singly rotating Myers-Perry-de Sitter (MP-dS) black holes against scalar field perturbations. The phase spaces of MP-dS black holes with one spin parameter are discussed. Additionally, the quasinormal modes (QNMs) of MP-dS black holes are calculated via the asymptotic iteration method and sixth-order Wentzel-Kramers-Brillouin approximation. For near-extremal MP-dS black holes, the event horizon may be considerably close to the cosmological horizon. In such cases, the Pöschl-Teller technique yields an accurate analytic formula for the QNMs. It is found that when the spin parameter of a black hole increases, the scalar perturbation modes oscillate at higher frequencies and decay faster. Furthermore, the MP-dS black hole with a single rotation is found to be stable under perturbation.

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arXiv2020

Weak Cosmic Censorship in Kerr-Sen Black Hole under Charged Scalar Field

We investigate the weak cosmic censorship conjecture for Kerr-Sen black holes, which are solutions to the four-dimensional low-energy effective field theory for the heterotic string theory, based on the scattering of a charged scalar field. When the fluxes of the scalar field are assumed to transfer its conserved quantities to the black hole, extremal and near-extremal black holes cannot be over-spun and over-charged in their first-order variations, which is sufficient to conclude that the weak cosmic censorship conjecture is valid for Kerr-Sen black holes. We confirm our conclusion by relating it to the first, second, and third laws of thermodynamics.

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arXiv2020

Thermodynamics in Rotating Anti-de Sitter Black Holes with Massive Scalar Field in Three Dimensions

We investigate the tendency in variations of the dual CFT$_2$ when a rotating AdS$_3$ black hole changes by fluxes transferred by the scattering of a massive scalar field according to the AdS/CFT correspondence. The conserved quantities of the black hole are definitely constrained by the extremal condition. Moreover, the laws of thermodynamics provide a direction for the changes in the conserved quantities, so the black hole cannot be extremal under the scattering. This is naturally preferred. By the relationship between the rotating AdS$_3$ black hole and dual CFT$_2$, we find that such changes in the black hole constrain variations in eigenstates of the dual CFT$_2$. Furthermore, the tendency in the variations is closely related to the laws of thermodynamics.

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arXiv2019

Quasinormal Modes of Massive Scalar Field with Nonminimal Coupling in Higher-Dimensional de Sitter Black Hole with Single Rotation

We analytically investigate the quasinormal modes of the massive scalar field with a nonminimal coupling in the higher-dimensional de Sitter black hole with a single rotation. According to the separated scalar field equation, the boundary conditions of quasinormal modes are well constructed at the outer and cosmological horizons. Then, under near-extremal conditions, where the outer horizon closes to the cosmological horizon, the quasinormal frequencies are obtained and generalized to universal form in the higher-dimensional spacetime. Here, the real part of the frequency includes the scalar field contents, and its imaginary part only depends on the surface gravity at the outer horizon of the black hole.

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arXiv2019

Charged Particle and Strong Cosmic Censorship in Reissner-Nordström-de Sitter Black Holes

We investigate the instability of the unstable circular orbit of a charged null particle to test the strong cosmic censorship conjecture in Nariai-type near-extremal Reissner--Nordström--de Sitter black holes. The instability is estimated as the Lyapunov exponent and found to depend on the mass and charge of the black hole. Then, we explicitly show that charged null particles in unstable circular orbits correspond to the charged massless scalar field in the eikonal limit. This provides a compact relationship representing the quasinormal frequency in terms of the characteristics of unstable circular orbits in near Nariai-type extremal conditions. According to this relationship, the strong cosmic censorship conjecture is valid.

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arXiv2019

Strong Cosmic Censorship under Quasinormal Modes of Non-Minimally Coupled Massive Scalar Field

We investigate the strong cosmic censorship conjecture in lukewarm Reissner-Nordström-de Sitter black holes (and Martínez-Troncoso-Zanelli black holes) using the quasinormal resonance of non-minimally coupled massive scalar field. The strong cosmic censorship conjecture is closely related to the stability of the Cauchy horizon governed by the decay rate of the dominant quasinormal mode. Here, dominant modes are obtained in the limits of small and large mass black holes. Then, we connect the modes by using the WKB approximation. In our analysis, the strong cosmic censorship conjecture is valid except in the range of the small-mass limit, in which the dominant mode can be assumed to be that of the de Sitter spacetime. Particularly, the coupling constant and mass of the scalar field determine the decay rate in the small mass range. Therefore, the validity of the strong cosmic censorship conjecture depends on the characteristics of the scalar field.

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arXiv2019

Charged Black Hole in Gravity's Rainbow: Violation of Weak Cosmic Censorship

We investigate the validity of weak cosmic censorship conjecture for electrically charged black holes in the presence of gravity's rainbow under charged particle absorption. The rainbow effect is shown to play an important role when the black hole is modified by a particle carrying energy and electric charge. Remarkably, we prove that the rainbow-charged black hole can be overspun beyond the extremal condition under charged particle absorption. Further, it is demonstrated that the second law of thermodynamics and cosmic censorship conjecture are violated owing to the rainbow effect.

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arXiv2019

Weak Cosmic Censorship with Pressure and Volume in Charged Anti-de Sitter Black Hole under Charged Scalar Field

We investigate the weak cosmic censorship conjecture in charged anti-de Sitter black holes with thermodynamic pressure and volume by the scattering of the charged scalar field in four and higher dimensions. We assume that the internal energy and electric charge of the black hole changes infinitesimally according to the energy and charge fluxes of the scalar field. Then, found to be an isobaric process, the changes in the black hole can be well reproduced to the first law of thermodynamics, although we find that the second law of thermodynamics is violated in extremal and near-extremal black holes. Nevertheless, the weak cosmic censorship conjecture remains valid, because the extremality of a black hole is invariant despite changes in internal energy and electric charge.

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arXiv2018

Weak Cosmic Censorship Conjecture in Kerr-(Anti-)de Sitter Black Hole with Scalar Field

We investigate the weak cosmic censorship conjecture in Kerr-(anti-)de Sitter black holes under the scattering of a scalar field. We test the conjecture in terms of whether the black hole can exceed the extremal condition with respect to its change caused by the energy and angular momentum fluxes of the scalar field. Without imposing the laws of thermodynamics, we prove that the conjecture is valid in all the initial states of the black hole (non-extremal, near-extremal, and extremal black holes). The validity in the case of the near-extremal black hole is different from the results of similar tests conducted by adding a particle because the fluxes represent the energy and angular momentum transferred to the black hole during the time interval not included in the tests involving the particle. Using the time interval, we show that the angular velocity of the black hole with the scalar field of a constant state takes a long time for saturation to the frequency of the scalar field.

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arXiv2018

Coalescence of Kerr Black Holes: Binary Systems from GW150914 to GW170814

We investigate the energy of the gravitational wave from a binary black hole merger by the coalescence of two Kerr black holes with an orbital angular momentum. The coalescence is constructed to be consistent with particle absorption in the limit in which the primary black hole is sufficiently large compared with the secondary black hole. In this limit, we analytically obtain an effective gravitational spin--orbit interaction dependent on the alignments of the angular momenta. Then, binary systems with various parameters including equal masses are numerically analyzed. According to the numerical analysis, the energy of the gravitational wave still depends on the effective interactions, as expected from the analytical form. In particular, we ensure that the final black hole obtains a large portion of its spin angular momentum from the orbital angular momentum of the initial binary black hole. To estimate the angular momentum released by the gravitational wave in the actual binary black hole, we apply our results to observations at the Laser Interferometer Gravitational-Wave Observatory: GW150914, GW151226, GW170104, GW170608, and GW170814.

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arXiv2017

Thermodynamics with Pressure and Volume under Charged Particle Absorption

We investigate the variation of the charged anti-de Sitter black hole under charged particle absorption by considering thermodynamic volume. The variation exactly corresponds to that expected as the first law of thermodynamics. Nevertheless, we find the decrease of the Bekenstein-Hawking entropy for extremal and near-extremal black holes under the absorption to be an irreversible process. This violation of the second law of thermodynamics is only found in the case considering thermodynamic volume. We test the weak cosmic censorship conjecture affected by the violation. Fortunately, the conjecture is still valid. However, extremal and near-extremal black holes do not change their configurations under the absorption. This is quite different from the case without thermodynamic volume.

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